1. Field of the Invention
The present invention relates to secondary battery packs such as lithium ion batteries and so forth.
2. Description of the Related Art
An electronic device that uses a battery pack requires several types of power supply voltages. Thus, the electronic device has a plurality of voltage converters. The voltage converters are for example DC-DC converters that raise and/or lower voltages and generate a plurality of desired voltages. Likewise, when a secondary battery pack is charged with electricity, an output voltage of a charger (AC adaptor) is raised and/or lowered by the DC-DC converters of the electronic device.
The following Patent Document 1 describes a battery pack that has a secondary battery and a voltage drop type converter and that outputs a constant voltage.
[Patent Document 1] Japanese Patent Laid-Open Publication No. HEI 7-7864
FIG. 4 shows the relation of a conventional battery pack and a main body of an electronic device (hereinafter simply referred to as electronic device). Reference numeral 1 represents a battery pack. Reference numeral 10 represents the electronic device. The electronic device 10 has a discharging circuit 11. The battery pack 1 has a secondary battery 2, a protecting circuit (IC circuit) 3, a discharging controlling FET 4, a charging controlling FET 5, and parasitic diodes 6 and 7 for the FETs 4 and 5.
The secondary battery 2 is for example a lithium ion battery, a lithium polymer battery, a nickel hydrogen battery, a nickel cadmium battery, a lithium metal battery, or the like. The lithium ion battery is formed in for example a box shape. The whole secondary battery 2 is housed by a steel battery case. The lithium polymer battery is sealed with an aluminum laminate film.
Since the lithium ion battery cannot stand an overcurrent and overdischarging, a battery cell and a protecting circuit are integrated into a battery pack. The protection circuit 3 have three protecting functions against overcharging, overdischarging, and overcurrent. Next, these protecting functions will be described in brief.
Next, the function of protection against overcharging will be described. When the lithium ion battery is charged with electricity, the battery voltage continuously rises even after the battery is fully charged. When the lithium ion battery is overcharged, it may become dangerous. Thus, the lithium ion battery should be charged with a constant current and a constant voltage so that the charging control voltage is equal to or lower than the rating of the battery (for example, 4.2 V). However, since the charger may become defective and a different type charger may be used, the lithium ion battery may be overcharged. Thus, when the lithium ion battery is overcharged and the battery voltage becomes a predetermined voltage or higher, the protecting circuit turns off the charging controlling FET 5 so as to shut out the charging current. This function is the protecting function against overcharging.
Next, the protecting function against overdischarging will be described. When the lithium ion battery is discharged below the rated discharge stop voltage and becomes an overdischarged state in which the battery voltage becomes for example 2 V to 1.5 V or below, the lithium ion battery may become defective. When the lithium ion battery is discharged and the battery voltage becomes a predetermined voltage or below, the protecting circuit turns off the discharging controlling FET 4 and shuts out the discharging current. This function is the protecting function against overdischarging.
Next, the protecting function against overcurrent will be described. When the plus terminal and the minus terminal of the lithium ion battery are short-circuited, there is a risk of which a large current flows in the battery and it is abnormally heated up. When the discharging current that flows exceeds a predetermined current value, the protecting circuit turns off the discharging controlling FET 4 and shuts out the discharging current. This function is the protecting function against overcurrent.
A discharging DC-DC converter 12 and a charging DC-DC converter 13 of the electronic device 10 are connected to external terminals of the foregoing battery pack 1. The discharging DC-DC converter 12 stabilizes the battery voltage that is input from the external terminals and supplies the stabilized voltage to the discharging circuit 11 through a discharging terminal 14 and a ground terminal 15.
A charging voltage is supplied from a charger 17 to the charging DC-DC converter 13 through a charging terminal 16 and the ground terminal 15. The charging DC-DC converter 13 converts the charging voltage supplied from the charger 17 into a rated charging voltage and supplies the rated charging voltage to the secondary battery 2 of the battery pack 1 through the external terminals.
FIG. 5 shows another example of the structure of a conventional battery pack and an electronic device 10. The structure shown in FIG. 5 is the same as the structure of the battery pack shown in FIG. 4 except that a battery pack 1 contains a charging DC-DC converter 12.
FIG. 6 shows another example of the structure of a conventional battery pack and an electronic device 10. The structure shown in FIG. 6 is the same as the structure shown in FIG. 4 except that a battery pack 1 contains a discharging DC-DC converter 13.
As described above, since the conventional secondary battery pack needs two DC-DC converters, which are the charging DC-DC converter 12 and the discharging DC-DC converter 13. Thus, the conventional secondary battery pack is adversely expensive and needs the space for them. In addition, since the electronic device has the DC-DC converter, there is a risk of which the DC-DC converter generates noise. Thus, it was necessary to consider a circuit against the noise.